1. Field of the Invention
The present invention relates to a combustion chamber structure of a spark ignition-type internal combustion engine.
2. Description of the Related Art
A combustion chamber structure of an internal combustion engine, in which a squish area is created between a piston top surface and the lower surface of a cylinder head forming the combustion chamber, and a squish flow is generated in the intake air (air or air-fuel mixture) existing in the squish area at around the compression top dead center of the piston to promote combustion, has been employed. The squish flow functions not only to stir the intake air but also to transfer the intake air toward a spark plug disposed close to the center of the combustion chamber. As the squish flow contains a lot of downward components with respect to a squish surface, however, when the squish surface is formed in a horizontal position (that is, orthogonal to the axis of the piston), it is unlikely that the squish flow is sufficiently transferred toward the spark plug that is positioned slightly above the squish surface in the combustion chamber.
In consideration of the above-mentioned circumstances, a combustion chamber structure designed to produce a squish flow in an obliquely upward direction has been suggested, for example, in Unexamined Japanese Patent Publication 10-184366 (hereinafter referred to as Patent Document 1). In the combustion chamber structure disclosed in Patent Document 1, there is formed a conical squish surface in the rim of a piston top surface, and a squish flow is created to be sent to the spark plug in between the squish surface and the lower surface of a cylinder head forming a pent roof-shaped combustion chamber from the rim side of the combustion chamber toward the center of the combustion chamber when viewed in plan view and obliquely upward when viewed in side view.
In this combustion chamber structure, the squish surface on the piston side has a conical shape, while the lower surface of the cylinder head is made up mainly of two plane surfaces inclined to the intake side and the exhaust side in order to form the pent roof-shaped combustion chamber. Therefore, it is only a very small region where a space suitable for the generation of a squish flow at around the compression top dead center of the piston is formed. If apart from the region slightly in the circumferential direction, the space is enlarged and does not contribute to the generation of the squish flow. For this reason, it is impossible to produce a strong squish flow, so that the transfer of the squish flow to the spark plug is consequently insufficient. It is then hard to say that the above combustion chamber structure is a credible improvement.